1. Field of the Invention
The present invention relates to a projection optical system, and more specifically to a projection optical system that is mounted in a projector having, for example, a digital micromirror device or a LCD (liquid crystal display), as a display device and that obliquely performs enlargement projection of an image formed on the surface of the display device onto the screen surface.
2. Description of Related Art
As an image display for use in digital satellite broadcasts at home and home theater, low-cost and high-performance projectors having a digital micromirror device or a LCD as a display device have been becoming increasingly widespread in recent years. Thus, there has been a great demand for a slim rear projector that does not occupy too much space even in a small room, which requires a wider-angle projection optical system. Moreover, the volume of information displayed has been considerably larger than before, and the display device has been provided with an even higher resolution than before accordingly, thus leading to a demand for smooth, clear, and high-quality images.
To achieve a low-cost rear projector with a high resolution, so-called pixel shift has been conventionally practiced by which an image projected on the screen surface is shifted by a small degree (for example, one half of a pixel) so as to increase the apparent number of pixels. The practice of pixel shift causes pixels to be superimposed on one another, which makes the contour of the pixels projected on the screen surface less outstanding, thereby permitting display of a smooth image. Moreover, due to its ability to increase the volume of information displayed on the screen surface without changing the number of pixels of the display device, the pixel shift is very effective in achieving a higher-grade image through an improvement in the resolution. As a projector employing this pixel shift, Patent publications 1 and 3 propose those which achieve pixel shift by way of slightly decentering a mirror while Patent publication 2 proposes one which achieves pixel shift by way of slightly decentering a flat glass.
[Patent publication 1] Japanese Patent Application Laid-open No. H4-319937
[Patent publication 2] U.S. Pat. No. 5,237,399
[Patent publication 3] Japanese Patent Application Laid-open No. H7-49477
However, with the projector constructions proposed in Patent publications 1 to 3, image projection is performed with an optical power possessed by a refractive optical system; therefore, an increase in the number of components and an increase in the back focus as a result of adding a mirror or a flat glass leads to upsizing of the entire apparatus and a cost increase. Especially in a case where a mirror or a flat glass to be slightly decentered is large in size, this results in upsizing of mechanisms such as a holding mechanism, a decentering mechanism, and the like, and thus requires complicated mechanisms for achieving slight decentering with even more accuracy, which contributes a further cost increase and upsizing. Further, the projector proposed in Patent document 2 suffers from chromatic aberration occurring with the flat glass to be slightly decentered.